What type of Differential oil?

Hello I have a 65 dart with a904 trans and 8.25 (8 1/4) suregrip rearend from a 78 aspen.
I only know the rearend is a 8.25 has suregrip and 3.21 gear ratio with large bolt pattern.

1. What type of fluid should go into this rearend?
   1. I was reading and it seems some say don't use synthetic only use conventional 80w-90 with friction modifier from ford or Mopar
   2. Some say use conventional "hypoid" gear oil 80w90 with friction modifier (BTW what is "hypoid" gear oil?)
   3. Other say synthetic is fine with friction modifier
   4. some say it depends if its a cone or clutch type rearend? (how can I check which it is?)
   5. And still others say 85w-140 conventional gear oil with friction modifier.

Does anyone have a definitive answer? Does anyone have a manual for an old aspen or 70s Mopar that recommends a specific gear oil.
The original 64 Dart manual I have also mentions "hypoid" sae90 lubricant; but this is not a 64 suregrip differential.

Thank you.